I've built quite a few over the years and it's fairly easy, but you do have to consider the loads and where on the beam they'll be. I'd skip the aluminum beam all together and just use wood. Aluminum flexes way too much, though it is light.
I usually make box beams for the side posts, rather then a single length of wood, as they're stronger. I'd also do a 4 legged kind of "A" frame sided thing, instead of a inverted "T" side leg too. Steel is a handy material for these things if you have some welding skills.
If it was me, I'd make an 8" or 10" wooden "I" beam as the cross piece. A 2x8 or 2x10 center (web) and 2x4 flanges on each side. This is incredibly stiff and strong. It's stronger if the 2x8 is rabbeted slightly into the 2x4's (1/4 to 3/8" deep groove), naturally all glued and screwed together. The legs I'd make from more of the same, except they don't need to be an "I" beam, just triangulated together ("A" frame style).
The wider the gantry, the bigger the cross beam needs to be. The major issues with gantries are wracking, meaning it might want to lean one way or the other, depending on how stout the legs are. Gussets at the top of the legs, to the cross beam help, but make them big. Making it roll around is another problem, requiring it to be even stiffer. If just for turning a hull, some 2x8 A frame legs, 3/4" plywood gussets and the wooden I beam will do. If wanting to roll around the boat too, then you should tie the two A frames together at the bottom and the cross beam. Get wheels big enough to handle (each) the full load with a 20% safety margin. The bigger the diameter of the wheel, the easier it is to roll around too.